[omginternets]

Jtsummers' explanation is correct, but I wanted to weigh in with an easy rule of thumb, which you may find easier to remember:

1. A law gives you a relationship with no mechanism. It almost always appears as a mathematical formula. You know how the pieces change with respect to another, but not why.

2. A theory gives you a mechanism. It tells you why. On rare occasions, it will not provide quantifiable predictions, in which case it is a qualitative theory.